The present invention relates to a carrier to be mixed with a polymer toner to provide a two-component developer for electrophotography and a developer containing the carrier.
A two-component dry developer for electrophotography comprises a toner and a carrier. A carrier is mixed with a toner in a mixing zone of a developing machine to give a desired charge quantity to the toner and carries the charged toner to an electrostatic latent image formed on a photoreceptor to form a toner image. The developer is replenished with a supplementary amount of a fresh toner for repeated use.
Applications of the developer of this type have been diversified with the wide spread of electrophotographic equipment such as copiers, facsimiles, and printers. In particular, market demands for higher image quality and a longer developer life have been increasing.
To meet the demand for higher image quality, reduction of a toner particle size has been proposed. Since toner particle size reduction tends to be accompanied by reduction of chargeability, it is necessary for the carrier to have a reduced particle size to gain in specific surface area for imparting a sufficient charge quantity to the toner. However, a developer comprising a smaller size toner and a smaller size carrier has poorer flowability to have a slower rise of charge, which can cause such problems as toner scattering.
Intensified mixing has been suggested to improve the flowability, which increases the stress on the developer. An increased stress will induce a spent-toner phenomenon as it is called (adhesion of a toner to the surface of the carrier particles) and cause the resin coat to fall off the carrier core, thereby accelerating deterioration of the developer. As a result, the developer cannot maintain satisfactory developing performance for a long period of time. Additionally, reduction of toner""s particle size results in reduction of production yield, leading to an increase of cost.
To overcome these problems, polymer toners have recently been under development. Produced by polymerization methods involving no grinding step, polymer toners with small particle sizes can be obtained in good yield and, having rounded shapes, exhibit satisfactory flowability even with small particle sizes. Besides, compared with ground toners, polymer toners have sharper particle size distributions and are therefore fit for high quality imaging. However, because polymer toners are produced by polymerization in an aqueous solvent in the presence of a large quantity of a dispersant having a polar group, their charging characteristics largely fluctuate with environmental variations as compared with ground toners.
A number of developers have hitherto been proposed aiming at settlement of the environment dependence issue.
JP-A-7-301958 proposes a carrier coated with a coating agent containing a specific charge control agent for improving environmental stability. However, where the coated carrier is mixed with a polymer toner, the resulting developer undergoes reduction of charge especially under a high temperature high humidity condition, failing to fulfill the demand for environmental stability. The publication also suggests that the coverage of the coating film is increased above a certain level to reduce carrier adhesion. Where combined with a polymer toner, however, it is difficult to achieve reduction of carrier adhesion consistent with improvement on environmental stability.
JP-A-8-62899 discloses use of a mixture of two carriers showing different changes with the environment. Mixing two kinds of carriers seemingly improves environmental stability but, in fact, results in a broadened charge distribution with environmental variations, which can cause fog, selective development with a toner, and the like. That is, the technique is insufficient for maintaining high image quality.
JP-A-7-287422 mentions that existence of 0.1 to 5% by number of metal atoms on the surface of silicone-coated carrier particles is effective in stabilizing charging to environmental variations and accelerating a rise of charge. JP-A-11-295934 teaches that existence of 7 to 20% by number of metal atoms, such as iron and alkali metals, on the surface of carrier particles prevents charges from being accumulated thereby to provide images stably even in a low temperature low humidity condition. However, developers comprising these carriers are liable to leak electricity in a high temperature high humidity condition, resulting in a failure to secure a desired charge quantity and causing toner scattering and fog.
JP-A-63-243962 proposes a developer comprising a specific polymer toner and a spherical carrier. According to the disclosure, the shear force imposed by carrier particles on toner particles can be reduced by using a spherical carrier and controlling the toner particle size and the carrier particle size within the respective specific ranges. The toner particles are thus prevented from being destroyed and exposing the low-softening component on their broken surface. As a result, toner""s filming on the sleeve can be prevented. However, considering that the fluctuation in charge quantity with environmental variation is admittedly attributed to the interactions between moisture in the air and a developer, the balance between leakage and accumulation of charges could not be stabilized to environmental changes merely by using spherical carrier particles. That is, the proposed developer does not sufficiently meet the demand for environmental stability.
JP-A-8-76407 proposes controlling the concentration of a toner and the specific gravity and the average particle size of a toner and a carrier to improve environmental stability of a polymer toner. Seeing that the environment dependence is decided chiefly by the mutual action among a toner, a carrier, and the moisture content of the environment, the above proposal alone results in a failure to retain high image quality.
As reviewed above, a large amount of a dispersant having a polar group is used in the polymerization system comprising an aqueous solvent for the preparation of polymer toners. Although the produced polymer particles are worked up by washing, drying or other means, a water content or the dispersant inevitably remain in and/or on the particles. Probably because such residual substances interact with the moisture content in the air, polymer toners show remarkable fluctuations of electrical characteristics with variations of environmental conditions, such as temperature and humidity, compared with ground toners.
In a high temperature high humidity condition, for example, the toner charge will reduce to cause toner scattering and fog and, besides, charges are apt to leak to destroy an electrostatic latent image, or the resistance of the developer tends to reduce to cause carrier adhesion.
In a low temperature low humidity condition, on the other hand, the toner charge will increase to reduce the image density. In case of an extreme increase in charge quantity, the carrier is dragged with the polymer toner being transferred to a photoreceptor, resulting in carrier adhesion. The resistance of the developer also increases, resulting in a reduced effective bias, which can cause image density reduction and fog.
While the causes of charge variations with environmental variation have not necessarily been elucidated, destruction of the balance among (i) the charge transfer rate between a carrier and a toner, (ii) the degree of charge accumulation, and (iii) the degree of charge leakage by a moisture content is one of the causes.
Increasing carrier""s resistance too much in an attempt to prevent charge leakage in a high temperature high humidity condition would result in reduction of developing ability (failure of obtaining a sufficient image density) and would accelerate charge accumulation in a low temperature low humidity condition. If the resistance of a carrier or a toner is decreased in an attempt to suppress excessive charge accumulation in a low temperature low humidity condition, fogging on a drum due to carrier adhesion or charge injection can result, and charge leakage in a high temperature high humidity condition would be accelerated.
An object of the present invention is to provide a carrier to be mixed with a polymer toner to make an electrophotographic developer which has reduced environment dependence of charge quantity to assure satisfactory imaging performance with no carrier adhesion under broad environmental conditions from a low temperature low humidity condition to a high temperature high humidity condition.
Another object of the present invention is to provide a developer containing the carrier.
The present inventors have extensively studied on charge stability of a developer comprising a polymer toner against environmental variations. As a result, they have found it important for obtaining environmental stability that the balance between charge accumulation and leakage should be stable to environmental variation. They have found that a resin-coated carrier having an optimum area of its core exposed provides a developer which achieves charge stabilization for an extended period of time while retaining high quality imaging characteristics free of image defects such as carrier adhesion.
The present invention provides a resin-coated carrier to be mixed with a polymer toner obtained by suspension polymerization or emulsion polymerization to provide an electrophotographic developer, which exposes 2 to 20% of the surface area of the core thereof, the average exposed area ratio per exposed part being 0.03% or less.
The present invention also provides an electrophotographic developer comprising the above-described carrier and a polymer toner obtained by suspension polymerization or emulsion polymerization.
The carrier and the developer according to the present invention hold a good balance between charge leakage in a high temperature high humidity condition and charge accumulation in a low temperature low humidity condition thereby to retain stable charging characteristics against environmental changes while exhibiting high quality imaging characteristics free from image defects, such as carrier adhesion, for a prolonged period of time.